The present invention relates to a rubber composition with a specific composition. Vulcanized rubber made of the rubber composition of the present invention shows low rolling resistance and excellent wet skid resistance, as well as satisfactory wear resistance, tensile strength and the like, and is particularly useful for tire treads.
Recent requirements for lower fuel consumption by automobiles have created a need for a composition such as conjugated diene-based rubber compositions whose rubbers have low rolling resistance, excellent wear resistance and fracture properties, and high wet skid resistance as a representative indicator of good driving stability.
The tire rolling resistance can be reduced by lowering hysteresis loss of the vulcanized rubber. This hysteresis loss can be evaluated based on various properties. For example, a composition having high impact resilience at 50 to 80xc2x0 C., low tan xcex4 at 50 to 80xc2x0 C. or low Goodrich heat release, is preferable. A raw rubber having low hysteresis loss include natural rubber, isoprene rubber, butadiene rubber and the like, but these are also associated with the problem of low wet skid resistance.
On the other hand, there have been proposed a method of using an inorganic filler such as silica and the like as a reinforcing agent, or a method of combining an inorganic filler and carbon black in recent years. Tire treads employing an inorganic filler or both an inorganic filler and carbon black show low rolling resistance and excellent driving stability represented by wet skid resistance. However, they exhibit a problem of poor wear resistance, tensile strength and the like for the vulcanized rubber. The cause is believed to be the fact that the affinity of the inorganic filler for the conjugated diene-based rubber is lower than that of the carbon black, such that a sufficient reinforcing effect cannot be achieved.
In order to increase the affinity between the inorganic filler and the conjugated diene-based rubber, it has been attempted to use a conjugated diene-based rubber introduced a functional group with affinity for an inorganic filler therein. For example, there have been proposed a conjugated diene-based rubber introduced a hydroxyl group therein (WO96/23027), a conjugated diene-based rubber introduced an alkoxysilyl group therein (JP-A-9-208623) and a conjugated diene-based rubber introduced an alkoxysilyl group and an amino and/or a hydroxyl group therein (JP-A-9-208633). However, most conjugated diene-based rubbers introduced such functional group exhibit strong interaction with the inorganic filler when the inorganic filler is combined therewith, and this creates such problems as impaired dispersion of the inorganic filler, greater heat release during processing, poor proccessability and the like.
It is an object of the invention to overcome the problems referred to above by providing a rubber composition which can give a vulcanized rubber having low rolling resistance and excellent wet skid resistance, as well as satisfactory wear resistance and tensile strength, which is useful for automobile tire treads and the like.
A rubber composition for formation of a tire tread and the like usually contains a reinforcing agent for the purpose of improving tensile strength and wear resistance, but an inorganic filler such as silica is not easily dispersed uniformly because of their tendency to aggregate. Using a rubber composition in which a reinforcing agent is not uniformly dispersed leads to a result that a desired effect of including the reinforcing agent cannot be achieved, as well as that the proccessability is also notably impaired. For this reason, it has generally been attempted to improve the dispersion properties by formulating a silane coupling agent when using a silica as an inorganic filler. However, it was found that inclusion of fine particles composed of crosslinked rubber having a specific composition in a rubber composition can adequately improve tensile strength even with a small amount of a silane coupling agent or even without including a silane coupling agent, and that the resulting rubber composition has excellent proccessability.
The present invention is based on the findings described above and can be described as follows.
A rubber composition of the first aspect of the invention is characterized in that it comprises [1A] a crosslinked rubber particle containing, as repeating units, (a1) 40 to 79.99% by weight of a conjugated diene monomer unit, (a2) 20 to 50% by weight of an aromatic vinyl monomer unit and (a3) 0.01 to 10% by weight of a monomer unit formed by a monomer having at least two polymerizable unsaturated groups with respect to 100% by weight of the total of (a1), (a2) and (a3), and [2A] a conjugated diene/aromatic vinyl copolymeric rubber wherein vinyl bond content of the conjugated diene unit is 10 to 30% by weight and 1,4-trans bond content is exceeding 55% by weight.
A rubber composition of the second aspect of the invention is characterized in that it comprises [1B] a crosslinked rubber particle containing, as repeating units, (b1) 40 to 99.89% by weight of a conjugated diene monomer unit, (b2) 0 to 50% by weight of an aromatic vinyl monomer unit, (b3) 0.01 to 10% by weight of a monomer unit formed by a monomer having at least two polymerizable unsaturated groups and (b4) 0.1 to 30% by weight of a monomer unit formed by a monomer having one polymerizable unsaturated group and at least one functional group selected from the group consisting of carboxylic group (CO2H and/or CO2xe2x88x92), hydroxyl group and epoxy group with respect to 100% by weight of the total of (b1), (b2), (b3) and (b4), and [2B] a conjugated diene/aromatic vinyl copolymeric rubber wherein vinyl bond content of the conjugated diene unit is 10 to 30% by weight and 1,4-trans bond content is exceeding 55% by weight.
The crosslinked rubber particle according to the first aspect of the invention is prepared by polymerizing a conjugated diene monomer forming a conjugated diene monomer unit (a1), an aromatic vinyl monomer forming an aromatic vinyl monomer unit (a2), and a monomer having at least two polymerizable unsaturated groups.
And the crosslinked rubber particle according to the second aspect of the invention is prepared by polymerizing a conjugated diene monomer forming a conjugated diene monomer unit (b1), an aromatic vinyl monomer forming an aromatic vinyl monomer unit (b2) as necessary, a monomer having at least two polymerizable unsaturated groups, and a monomer having one polymerizable unsaturated group and at least one functional group selected from the group consisting of carboxylic group (CO2H and/or CO2xe2x88x92), hydroxyl group and epoxy group.
In both first and second aspects of the invention, another polymerizable unsaturated monomer described hereunder may also be used.
As the conjugated diene monomer forming the conjugated diene monomer unit (a1) and (b1) described above, there may be used 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, chloroprene and the like. Any of these monomers may be used alone or in combination of two or more, respectively.
The content of the conjugated diene monomer unit (a1) according to the first aspect of the invention is 40 to 79.99% by weight and preferably 55 to 79.99% by weight. The content of the conjugated diene monomer unit (b1) according to the second aspect of the invention is 40 to 99.89% by weight and preferably 55 to 99.89% by weight. If the content of the conjugated diene monomer unit described above is under the respective lower limits, proccessability is not improved and tensile strength of the vulcanized rubber is lowered.
As the aromatic vinyl monomer forming the aromatic vinyl monomer unit (a2) and (b2) described above there may be used styrene, 2-methylstyrene, 3-methylstyrene, 4-methylstyrene, xcex1-methylstyrene, 2,4-dimethylstyrene, 2,4-diisopropylstyrene, 4-tert-butylstyrene, tert-butoxystyrenes and the like. These monomers may be used alone or in combination of two or more, respectively.
The content of the aromatic vinyl monomer unit (a2) according to the first aspect of the invention is 20 to 50% by weight and preferably 20 to 45% by weight. The content of the aromatic vinyl monomer unit (b2) according to the second aspect of the invention is 0 to 50% by weight and preferably 0 to 45% by weight. If the content of the aromatic vinyl monomer unit described above is exceeding 50% by weight, impact resilience of the vulcanized rubber is reduced, and the value of tan xcex4 of the vulcanized rubber at 50xc2x0 C. tends to be larger.
As the monomer having at least two polymerizable unsaturated groups forming the monomer unit (a3) and (b3) described above there may be used ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, trimethylolpropane di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, divinylbenzene, diisopropylbenzene, trivinylbenzene and the like. Among these, divinylbenzene and the like are preferred. Any of these monomers may be used alone or in combination of two or more, respectively.
The content of the monomer unit (a3) and (b3) having at least two polymerizable unsaturated groups described above according to both first and second aspects of the invention is 0.01 to 10% by weight and preferably 0.1 to 10% by weight, respectively. If the content of (a3) or (b3) is less than 0.01% by weight, proccessability of the rubber composition of the invention is reduced. On the other hand, if the content is greater than 10% by weight, not only proccessability of the rubber composition is impaired, but also tensile strength of the vulcanized rubber is greatly reduced, to an undesirable level.
As the monomer having one polymerizable unsaturated group and a carboxylic group (CO2H and/or CO2xe2x88x92) for the monomer unit (b4) according to the second aspect of the invention, there may be used
{circle around (1)} unsaturated carboxylic acid such as (meth)acrylic acid, maleic acid, fumaric acid, itaconic acid, tetraconic acid, cinnamic acid and the like, and
{circle around (2)} free carboxyl group-containing ester such as monoester of non-polymerizable polyvalent carboxylic acid such as phthalic acid, succinic acid and adipic acid with hydroxyl group-containing unsaturated compound such as (meth)allyl alcohol and 2-hydroxyethyl (meth)acrylate, and their salts and the like.
Among these, unsaturated carboxylic acids (especially (meth)acrylic acid and the like) are preferred. Any of these monomers having carboxylic group listed above may be used alone or in combination of two or more.
As the monomer having one polymerizable unsaturated group and a hydroxyl group according to the second aspect of the invention, there may be used
{circle around (1)} hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate and the like,
{circle around (2)} mono (meth)acrylates of a polyalkylene glycol (the number of alkylene glycol units is for example 2 to 23) such as polyethylene glycol, polypropylene glycol and the like,
{circle around (3)} hydroxyl group-containing unsaturated amides such as N-hydroxymethyl (meth)acrylamide, N-(2-hydroxyethyl) (meth)acrylamide, N,N-bis(2-hydroxyethyl) (meth)acrylamide and the like,
{circle around (4)} hydroxyl group-containing aromatic vinyl compounds such as o-hydroxystyrene, m-hydroxystyrene, p-hydroxystyrene, o-hydroxy-xcex1-methylstyrene, m-hydroxy-xcex1-methylstyrene, p-hydroxy-xcex1-methylstyrene, p-vinylbenzyl alcohol and the like, and
{circle around (5)} hydroxyl group-containing compounds such as (meth)allyl alcohol and the like.
Among these, hydroxyalkyl (meth)acrylates and hydroxyl group-containing aromatic vinyl compounds are preferred. Any of these monomers may be employed alone or in combination of two or more.
As the monomer having one polymerizable unsaturated group and an epoxy group according to the second aspect of the invention, there may be used epoxy group-containing compounds such as (meth)allylglycidylether, glycidyl (meth)acrylate, 3,4-oxycyclohexyl (meth)acrylate and the like. Any of these monomers may be employed alone or in combination of two or more.
The monomer forming the monomer unit (b4) described above is preferably a monomer having a hydroxyl group and/or an epoxy group in view of impact resilience and wear resistance of a vulcanized rubber as well as proccessability of the rubber composition, especially a reduced shrinkage of an unvulcanized rubber composition.
The monomer forming the monomer unit (b4) may be selected for use as desired. And proccessability of the rubber composition of the invention and mechanical strength of the vulcanized rubber and the like can be even further improved by suitable combination of a crosslinked rubber particle and a functional group composed in the conjugated diene/aromatic vinyl copolymeric rubber described hereunder. A combination of two or more monomers having a carboxylic group, a hydroxyl group or an epoxy group may also be used. For example, a combination of a monomer having a carboxylic group and a monomer having a hydroxyl group may be used as the monomer forming the monomer unit (b4).
The content of the monomer unit (b4) described above is 0.1 to 30% by weight and especially 0.3 to 15% by weight preferred. If the content of the monomer unit (b4) is less than 0.1% by weight, the affinity between the crosslinked rubber particle and an inorganic filler is insufficient, and proccessability of the rubber composition may be reduced. On the other hand, if the content is greater than 30% by weight, the crosslinked rubber particle and an inorganic filler interact strongly with each other, and proccessability may be reduced.
The crosslinked rubber particle [1A] described above in the first aspect of the invention contains the monomer unit (a1) of 40 to 79.99% by weight (preferably 50 to 79.99% by weight), the monomer unit (a2) of 20 to 50% by weight (preferably 20 to 45% by weight), and the monomer unit (a3) of 0.01 to 10% by weight (preferably 0.1 to 10% by weight), respectively.
The crosslinked rubber particle [1B] described above in the second aspect of the invention contains the monomer unit (b1) of 40 to 99.89% by weight (preferably 50 to 99.89% by weight), the monomer unit (b2) of 0 to 50% by weight (preferably 0 to 45% by weight), the monomer unit (b3) of 0.01 to 10% by weight (preferably 0.1 to 10% by weight) and the monomer unit (b4) of 0.1 to 30% by weight (preferably 0.3 to 15% by weight), respectively. A combination of preferable contents of the monomer unit described above is not particularly limited.
In the second aspect of the invention, the content of the aromatic vinyl monomer unit (b2) described above is not particularly limited so long as it is within the range from 0 to 50% by weight. The content of the aromatic vinyl monomer unit (b2) described above may for example be 20 to 50% by weight (the content of the conjugated diene monomer unit is 40 to 79.89% by weight in this case), or 20 to 45% by weight (the content of the conjugated diene monomer unit is 40 to 79.89% by weight in this case), or less than 20% by weight, or 10% by weight or less, or also 0% by weight (thus is not contained). In any of these cases the monomer (b4) described above is preferably a monomer unit formed by a monomer having one polymerizable unsaturated group and at least one functional group selected from the group consisting of a hydroxyl group and an epoxy group. Especially, the content of the aromatic vinyl monomer unit (b2) can be 20 to 50% by weight, the content of the conjugated diene monomer unit (b1) can be 40 to 79.89% by weight, and the monomer unit (b4) can be a monomer unit formed by a monomer having one polymerizable unsaturated group and at least one functional group selected from the group consisting of a hydroxyl group and an epoxy group. And the content of the aromatic vinyl monomer unit (b2) can be 0% by weight, the content of the conjugated diene monomer unit (b1) can be 40 to 99.89% by weight, and the monomer unit (b4) can be a monomer unit formed by a monomer having one polymerizable unsaturated group and at least one functional group selected from the group consisting of a hydroxyl group and an epoxy group. A crosslinked rubber particle having such a composition may be a crosslinked polybutadiene modified with a functional group.
The monomer unit constituting the crosslinked rubber particle [1A] and [1B] includes the other monomer unit. The other polymerizable unsaturated monomer forming the other monomer unit is preferably a compound having one copolymerizable unsaturated group, and while there are no particular restrictions on its type, those having polar groups are preferred.
As such monomer described above there may be used (meth)acrylonitrile, vinylidene cyanide, vinyl chloride, vinylidene chloride, (meth)acrylamide, maleimide, methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, iso-propyl (meth)acrylate, n-butyl (meth)acrylate, iso-butyl (meth)acrylate, sec-butyl (meth)acrylate, tert-butyl (meth)acrylate, n-amyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl (meth)acrylate and the like. Any of these monomers may be used alone or in combination of two or more, respectively.
In both first and second aspects of the invention, the amount of the above-mentioned monomer to be used may be 0.1 to 5% by weight and especially 0.5 to 3% by weight with respect to 100% by weight of the total of the monomers forming the crosslinked rubber particle.
The particle comprising the crosslinked rubber described above is a particulate rubber with a preferable toluene insoluble content of 80% by weight or more. The toluene insoluble content is determined by immersing 1 g of the particle comprising the crosslinked rubber in 100 ml of toluene for 24 hours at room temperature and then measuring the weight of a solid content in the filtrate obtained by filtering with a 100 mesh-sized wire mesh. And the crosslinked rubber particle described above is a particulate rubber with a particle size of preferably 500 nm or less as measured using a laser particle analysis system (Model No. xe2x80x9cLPA-3100xe2x80x9d by Otsuka Electronics Co., Ltd.).
The crosslinked rubber particle may be produced by emulsion polymerization, suspension polymerization and the like using a radical polymerization initiator. It is preferably produced by emulsion polymerization from the standpoint of the particle size and the particle size uniformity.
The radical polymerization initiator for an emulsion polymerization includes an organic peroxide such as benzoyl peroxide, lauryl peroxide, tert-butyl hydroperoxide, cumene hydroperoxide, p-methane hydroperoxide, di-tert-butyl peroxide, dicumyl peroxide and the like. In addition, a diazo compound such as azobisisobutyronitrile, an inorganic peroxide such as potassium persulfate, and a redox catalyst such as a combination of any of these peroxides with ferrous sulfate and the like may also be used. Any of these radical polymerization initiators may be used alone or in combination of two or more.
A chain transfer agent such as mercaptans including tert-dodecylmercaptan, n-dodecylmercaptan and the like, carbon tetrachloride, thioglycols, diterpene, terpinolene, xcex3-terpinene and the like may also be used in combination.
An emulsifier employed in an emulsion polymerization may, for example, be an anionic surfactant, a nonionic surfactant, a cationic surfactant as well as an amphoteric surfactant and the like. A fluorine-based surfactant may also be employed. Any of these emulsifiers may be used alone or in combination of two or more.
A suspension stabilizer employed in a suspension polymerization may, for example, be polyvinyl alcohol, sodium polyacrylate, hydroxyethyl cellulose and the like. Any of these suspension stabilizers may be used alone or in combination of two or more.
In an emulsion or a suspension polymerization, monomers, a radical polymerization initiator and the like may be charged all to the reaction vessel at once, or they may be added continuously or intermittently over the period of the reaction. The polymerization may be performed in an oxygen-free reaction vessel at 0 to 80xc2x0 C. with changing temperature, stirring condition or the like as desired during the reaction. The polymerization may be of a continuous system or a batch system.
The conjugated diene/aromatic vinyl copolymeric rubber in both first and second aspects of the invention is not particularly restricted so long as it is a rubber comprising a monomer forming a conjugated diene monomer unit and a monomer forming an aromatic vinyl monomer unit. As the monomer forming the conjugated diene monomer unit, the above-mentioned monomer forming the monomer unit (a1) may be used, and as the monomer forming the aromatic vinyl monomer unit, the above-mentioned monomer forming the monomer unit (a2) may be used. The content of the aromatic vinyl monomer unit in the conjugated diene/aromatic vinyl copolymeric rubber is preferably 20 to 50% by weight and especially 22 to 48% by weight.
Further, a polymerizable unsaturated monomer which is copolymerizable with these monomers may be used as option. The polymerizable unsaturated monomer may be used (meth)acrylonitrile, vinylidene cyanide, vinyl chloride, vinylidene chloride, (meth)acrylamide, maleimide, methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, iso-propyl (meth)acrylate, n-butyl (meth)acrylate, iso-butyl (meth)acrylate, sec-butyl (meth)acrylate, tert-butyl (meth)acrylate, n-amyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl (meth)acrylate and the like. Any of these monomers may be used alone or in combination of two or more.
The conjugated diene/aromatic vinyl copolymeric rubber may be a conjugated diene/aromatic vinyl copolymeric rubber wherein at least one functional group selected from the group consisting of carboxylic group, amino group, hydroxyl group, epoxy group and alkoxysilyl group is introduced into its molecule. This can increase the affinity between an inorganic filler and the conjugated diene/aromatic vinyl copolymeric rubber. The functional group may be the same type as that introduced into the crosslinked rubber particle, or they may be a different from each other. These functional groups may be introduced by using a polymerizable unsaturated monomer which has a functional group and is copolymerizable with the monomer described above.
In the case a conjugated diene/aromatic vinyl copolymeric rubber which a functional group is introduced by using the functional group-containing monomer described above, is employed, the content of the monomer unit containing a functional group is preferably 0.1 to 30% by weight with respect to the total of the rubber component. If the content of any of these monomer units is less than 0.1% by weight, it is difficult in obtaining an effect of introduction of a functional group. And the content exceeding 30% by weight results in a strong interaction between a conjugated diene/aromatic vinyl copolymeric rubber and a silica, which may lead to reduction in proccessability.
In the case the crosslinked rubber particle is the one whose content of the aromatic vinyl monomer unit (b2) described above is zero, and the monomer unit (b4) is a monomer unit formed by a monomer having one polymerizable unsaturated group and at least one functional group selected from the group consisting of a hydroxyl group and an epoxy group, a functional group introduced into the conjugated diene/aromatic vinyl copolymeric rubber described above is at least one selected from the group consisting of a carboxylic group (CO2H and/or CO2xe2x88x92), an amino group, a hydroxyl group, an epoxy group and an alkoxysilyl group, a hydroxyl group and/or an epoxy group are preferred.
As a monomer having one polymerizable unsaturated group and at least one functional group selected from the group consisting of a carboxylic group (CO2H and/or CO2xe2x88x92), an amino group, a hydroxyl group, an epoxy group and an alkoxysilyl group, a monomer having the same type of carboxylic group, hydroxyl group or epoxy group may be used as for the production as that in the crosslinked rubber particle described above, and a monomer having an amino group or an alkoxysilyl group may be used.
As a monomer forming the monomer unit having one polymerizable unsaturated group and amino group, tertiary amino group-containing monomer is preferable and there may be mentioned
{circle around (1)} Dialkylaminoalkyl (meth)acrylates such as dimethylaminomethyl (meth)acrylate. diethylaminomethyl (meth)acrylate, 2-dimethylaminoethyl (meth)acrylate, 2-diethylaminoethyl (meth)acrylate, 2-dimethylaminoethyl (meth)acrylate, 2-diethylaminoethyl (meth)acrylate, 2-(di-n-propylamino) ethyl (meth)acrylate, 2-dimethylaminopropyl (meth)acrylate, 2-diethylaminopropyl (meth)acrylate, 2-(di-n-propylamino) propyl (meth)acrylate, 3-dimethylaminopropyl (meth)acrylate, 3-diethylaminopropyl (meth)acrylate, 3-(di-n-propylamino) propyl (meth)acrylate and the like,
{circle around (2)} N-dialkylaminoalkyl group-containing unsaturated amides such as N-dimethylaminomethyl (meth)acrylamide, N-diethylaminomethyl (meth)acrylamide, N-(2-dimethylaminoethyl) (meth)acrylamide, N-(2-diethylaminoethyl) (meth)acrylamide, N-(2-dimethylaminopropyl) (meth)acrylamide, N-(2-diethylaminopropyl) (meth)acrylamide, N-(3-dimethylaminopropyl) (meth)acrylamide, N-(3-diethylaminopropyl) (meth)acrylamide and the like,
{circle around (3)} Tertiary amino group-containing aromatic vinyl compounds, such as N,N-dimethyl-p-aminostyrene, N,N-diethyl-p-aminostyrene, dimethyl(p-vinylbenzyl)amine, di-ethyl(p-vinylbenzyl)amine, dimethyl(p-vinylphenethyl)amine, diethyl(p-vinylphenethyl),amine, dimethyl (p-vinylbenzyloxymethyl)amine, dimethyl-[2-(p-vinylbenzyloxy) ethyl]amine diethyl(p-vinylbenzyloxymethyl)amine, diethyl[2-(p-vinylbenzyloxy)ethyl]amine, dimethyl (p-vinylphenethyloxymethyl) amine, dimethyl [2-(p-vinylphenethyloxy) ethyl]amine, diethyl(p-vinylphenethyloxymethyl)amine, diethyl[2-(p-vinylphenethyloxy)ethyl]amine, 2-vinylpyridine, 3-vinylpyridine, 4-vinylpyridine and the like.
Among these listed above, a dialkylaminoalkyl (meth)acrylates and a tertiary amino group-containing aromatic vinyl compounds are preferred.
As a monomer forming the monomer unit having one polymerizable unsaturated group and alkoxysilyl group there may be mentioned (meth)acryloxymethyl trimethoxysilane, (meth)acryloxymethyl methyldimethoxysilane (meth)acryloxymethyl dimethylmethoxysilane, (meth)acryloxymethyl triethoxysilane, (meth)acryloxymethyl methyldiethoxysilane, (meth)acryloxymethyl dimethylethoxysilane, (meth)acryloxymethyl tripropylsilane, (meth)acryloxymethyl methyldipropoxysilane, (meth)acryloxymethyl dimethylpropoxysilane, xcex3-(meth)acryloxypropyl trimethoxysilane, xcex3-(meth)acryloxypropyl methyldimethoxysilane. xcex3-(meth)acryloxypropyl dimethylmethoxysilane, xcex3-(meth)acryloxypropyl triethoxysilane, xcex3-(meth)acryloxypropyl methyldiethoxysilane, xcex3-(meth)acryloxypropyl dimethylethoxysilane, xcex3-(meth)acryloxypropyl tripropylsilane, xcex3-(meth)acryloxypropyl methyldipropoxysilane, xcex3-(meth)acryloxypropyl dimethylpropoxysilane, xcex3-(meth)acryloxypropyl methyldiphenoxysilane, xcex3-(meth)acryloxypropyl dimethylphenoxysilane, xcex3-(meth)acryloxypropyl methyldibenzyloxysilane, xcex3-(meth)acryloxypropyl dimethylphenoxysilane and the like.
Any of monomers having these functional groups may be used alone or in combination of two or more. And monomers having different functional groups may be used in combination.
The conjugated diene/aromatic vinyl copolymeric rubber according to both first and second aspects of the invention may be a styrene-butadiene copolymeric rubber and the like. The vinyl bond content in the conjugated diene unit of the conjugated diene/aromatic vinyl copolymeric rubber is preferably 10 to 30% by weight, more preferably 12 to 25% by weight and 1,4-trans bond content is preferably exceeding 55% by weight, more preferably exceeding 60% by weight. The upper limit is usually about 85% by weight. If the vinyl bond content increases, impact resilience and wear resistance of the vulcanized rubber may be reduced. On the other hand, if the 1,4-trans bond content is small, tensile strength is reduced. By adjusting a microstructure within the range specified above, a vulcanized rubber which is excellent in these characteristics can be obtained.
In both first and second aspects of the invention, the content of the conjugated diene unit constituting the conjugated diene/aromatic vinyl copolymeric rubber is preferably 50 to 80% by weight, particularly 55 to 80% by weight. And the content of the aromatic vinyl monomer unit is preferably 20 to 50% by weight, particularly 20 to 45% by weight.
In the case the conjugated diene/aromatic vinyl copolymeric rubber is the one having functional groups introduced therein, the content of the conjugated diene unit constituting the conjugated diene/aromatic vinyl copolymeric rubber is preferably 40 to 79.9% by weight, particularly 50 to 77.9% by weight, and the content of the aromatic vinyl monomer unit is preferably 20 to 50% by weight, particularly 20 to 45% by weight.
If the content of the aromatic vinyl monomer unit is small, wet skid resistance and wear resistance of the vulcanized rubber may be reduced. On the other hand, if the content is large, impact resilience of the vulcanized rubber is reduced and the value of tan xcex4 of the vulcanized rubber at 50xc2x0 C. tends to be larger. By adjusting the content of the aromatic vinyl monomer unit within the range specified above, a rubber composition which is excellent in these physical characteristics can be obtained.
The conjugated diene/aromatic vinyl copolymeric rubber is a rubber with a preferable toluene insoluble content of 30% by weight or less determined by immersing 1 g of the copolymeric rubber in 100 ml of toluene for 24 hours at room temperature and then measuring the weight of a solid portion in the filtrate obtained by filtering with a 100 mesh-sized wire mesh, and with a preferable weight-average molecular weight of 100,000 to 2,000,000 as measured by GPC (gel permeation chromatography) in terms of polystyrene.
In both first and second aspects of the invention, the conjugated diene/aromatic vinyl copolymeric rubber described above may be produced by emulsion polymerization or suspension polymerization. The polymerization method is not particularly limited and emulsion polymerization is preferred.
The rubber compositions in both first and second aspects of the invention can be ones that the other rubber component is added. A diene-based rubber is preferably used as the rubber component and may be used natural rubber, isoprene rubber, styrene-butadiene copolymeric rubber, butadiene rubber, butyl rubber, ethylene-propylene-diene ternary copolymeric rubber and the like. Among these rubbers, natural rubber, styrene-butadiene copolymeric rubber and butadiene rubber are preferable in the view of the physical characterization of an unvulcanized rubber composition or a vulcanized rubber composition obtained and the ease of carrying out of acquisition of a raw rubber.
A filler is usually incorporated into the rubber composition of the present invention. The filler is not particularly restricted, there may be used a silica, an inorganic filler consisting of an inorganic compound as represented by the formula (I), carbon black, carbon black-silica dual phase filler and the like.
mM1.xSiOy.zH2Oxe2x80x83xe2x80x83(I)
[In the formula (I), M1 is at least one selected from the group consisting of Al, Mg, Ti, and Ca; any oxide of any one of the metals; or any hydroxide of any one of the metals; and m, x, y, and z are integers from 1 to 5, 0 to 10, 2 to 5, and 0 to 10, respectively.]
A rubber composition incorporated a reinforcing inorganic filler shows a remarkable effect of the invention.
Examples of the inorganic compounds of formula (I) include alumina monohydrate (Al2O3.H2O), aluminum hydroxide [Al(OH)3] such as gibbsite and bayerite, magnesium hydroxide [Mg(OH)2], magnesium oxide (MgO), talc (3MgO.4SiO2.H2O), attapulgite (5MgO.8SiO2.9H2O), titanium white (TiO2), titanium black (TiO2nxe2x88x921), calcium oxide (CaO), calcium hydroxide [Ca(OH)2], magnesium aluminum oxide (MgO.Al2O3), clay (Al2O3.2SiO2), kaolin (Al2O3.2SiO2.2H2O), pyrophyllite (Al2O3.4SiO2.H2O), bentonite (Al2O3.4SiO2.2H2O), aluminum silicate (for example. Al2SiO5, Al4.3SiO4.5H2O), magnesium silicate (for example, Mg2SiO4, MgSiO3), calcium silicate (Ca2.SiO4), calcium aluminum silicate (for example, Al2O3.CaO.2SiO2), and calcium magnesium silicate (CaMgSiO4), crystalline aluminum silicate containing hydrogen which corrects a charge such as a variety of zeolite, alkali metal, or alkali earth metal, and the like. The xe2x80x9cMxe2x80x9d is preferably aluminum. Particle sizes of these inorganic compounds are preferably 10 xcexcm or less, more preferably 3 xcexcm or less. Using an inorganic compound whose particle size is 10 xcexcm or less leads to good fracture properties and excellent wear resistance of the vulcanized rubber.
Among the reinforcing filler described above, silica, aluminum hydroxide, carbon black and carbon black-silica dual phase filler are preferred.
The silica used may be a silica commonly used as a white reinforcing additive for synthetic rubbers. While the type of a silica is not particularly limited, a wet process type white carbon (a precipitated silica described in JP-A-62-62838), a dry process type white carbon, a colloidal silica and the like can be employed. Among those listed above, a wet process type white carbon whose main component is silicic hydrate is preferred particularly. Any of these silica-based compounds may be employed alone or in combination of two or more. While the specific surface area of the silica is not particularly limited, a nitrogen absorption specific surface area (area determined by BET method in accordance with ASTM D3037-81) is preferably 50 to 400 m2/g, more preferably 50 to 220 m2/g, most preferably 70 to 220 m2/g, for the purpose of achieving a sufficient improvement in reinforcing performance, wear resistance, exothermic behavior and the like.
While the type of a carbon black or the like is not particularly limited, those which may be employed are a furnace black, an acetylene black, a thermal black, a channel black, a graphite, and the like. Among those listed above, a furnace black such as SAF, ISAF, ISAF-HS, ISAF-LS, IISAF-HS, HAF, HAF-HS, HAF-LS, FEF, and the like is preferred. Any of these carbon blacks may be employed alone or in combination of two or more.
While the nitrogen absorption specific surface area of a carbon black determined similarly to a silica is not particularly limited, it is preferably 5 to 200 m2/g, more preferably 50 to 150 m2/g, most preferably 80 to 130 m2/g, for the purpose of achieving a sufficient improvement in tensile strength and wear resistance of a vulcanized rubber and the like. While the DBP absorption level of the carbon black is not particularly limited as well, it is 5 to 300 ml/100 g, preferably 50 to 200 ml/100 g, more preferably 80 to 160 ml/100 g, for the purpose of a sufficient improvement in tensile strength and wear resistance and the like. A further improvement in wear resistance can be achieved by using as a carbon black a high structure carbon black described in JP-A-5-230290, whose cetyltrimethylammonium bromide absorption specific surface area is 110 to 170 m2/g and whose DBP (24M4DBP) oil absorption level after compressing under 165 MPa and four times repetitively is 110 to 130 ml/100 g.
The content of the reinforcing filler is preferably 20 to 200 parts, especially 30 to 180 parts, more especially 30 to 150 parts, with respect to 100 parts of the total of the crosslinked rubber particle and the conjugated diene-aromatic vinyl copolymeric rubber. Little content of the reinforcing filler leads to an insufficient reinforcing effect, resulting in a problematically reduced tensile strength and the like. On the other hand, much content leads to a reduced workability. By adjusting the content in the range described above, a rubber composition whose workability in unvulcanized state is excellent and whose physical characterization in vulcanization is excellent, can be obtained.
The rubber composition of the invention may also contain the other various components mentioned below in addition to the filler.
An extending oil such as aromatic-based process oils, naphthene-based process oils, paraffin-based process oils and other petroleum-based blended oils may also be incorporated. Preferred as the extending oil are aromatic-based and naphthene-based process oils. The amount of the extending oil to be incorporated is preferably 100 parts or less, more preferably 80 parts or less, most preferably 70 parts or less with respect to 100 parts of solid content of the rubber component. If the content is exceeding 100 parts, viscosity of the rubber composition may be reduced.
As a vulcanization accelerator there may be used aldehyde ammonia-based, guanidine-based, thiourea-based, thiazole-based and dithiocarbaminic acid-based ones, and these are preferably used at 0.5 to 15 parts and especially 1 to 10 parts with respect to 100 parts of the total of the crosslinked rubber particle, the conjugated diene/aromatic vinyl copolymeric rubber and the other rubber component. Also, sulfur is representative as a vulcanizing agent, but sulfur-containing compound, peroxide and the like may also be used. The vulcanizing agent such as sulfur, sulfur-containing compound, peroxide and the like is usually used as the sulfur content, at 0.5 to 10 parts and preferably 1 to 6 parts with respect to 100 parts as the total of the crosslinked rubber particle, the conjugated diene/aromatic vinyl copolymeric rubber and the other rubber component.
In addition, appropriate amounts of a silane coupling agent, a vulcanizing aid, an anti-aging agent, a working adjuvant, a softener, an inorganic filler such as zinc oxide, calcium carbonate and magnesium carbonate and the like may also be incorporated.
The rubber composition of the present invention and rubber products employing it may be manufactured in the following fashion.
First, a crosslinked rubber particle, a conjugated diene/aromatic vinyl copolymeric rubber, the other rubber component, an inorganic filler, a reinforcing filler, a softener and other additives are kneaded at a temperature in the range of 70 to 180xc2x0 C. using a kneading machine such as a Banbury mixer. The kneaded product is then cooled, the vulcanizing agent such as sulfur and vulcanizing accelerators are added using a Banbury mixer or mixing roll, and the mixture is molded into the desired shape. This is followed by vulcanization at a temperature in the range of 140 to 180xc2x0 C. to obtain the desired vulcanized rubber as the rubber product.
A vulcanized rubber obtained using the rubber composition according to the present invention exhibits excellent wet skid resistance, tensile strength, impact resilience and the like. It also has satisfactory proccessability, and therefore the rubber composition is useful as a rubber composition for tires, and is particularly suitable for tire treads.
A pneumatic tire obtained using the rubber composition of the present invention as its tread is excellent in gas mileage, steering stability, fracture properties and wear resistance. And the rubber composition gives a high producibility due to a satisfactory proccessability.
A pneumatic tire according to the invention is produced by a standard method. Thus, a rubber composition supplemented if necessary with various agents as described above is extruded as being unvulcanized in to a tread part, affixed onto a tire mold by a standard method to form a raw tire, which is then pressurized with heating in a vulcanizing machine, whereby obtaining a pneumatic tire according to the invention.
Not only air but also inert gas such as nitrogen may be used as a gas in the pneumatic tire according to the invention.